Echo chamber

Etymology

The literal echo chamber is an acoustic space, used in recording, where sound reflects and repeats. From the mid-20th century onward the phrase was borrowed as a metaphor for insular communication, and it became a fixture of media criticism in the internet era. The image is exact: an input comes back amplified and unchanged.

noun phrase · /EK-oh CHAYM-ber/
A social or media environment where the same beliefs circulate and reinforce each other because dissenting views are absent or excluded.
What it names
It explains how a group can grow more confident and more extreme simply by talking mostly to itself.
Example
The forum started as a debate space but hardened into an echo chamber once the moderators began banning anyone who pushed back.
Related idea
An echo chamber actively excludes and discredits outside views, whereas a filter bubble simply fails to show them to you; one is a social choice, the other an algorithmic default.